This invention relates to a sleeve bearing intended for use in four-stroke internal combustion engines, particularly as a connecting rod bearing.
As known by those skilled in the art, a clearance is provided for holding a film of lubricating oil between the inner peripheral surface of the bearing and the crankpin. According to the current technology concepts, an adequate choice of the assembly clearance between the shaft and the bearing is made based on load and lubrication considerations, generally with the aid of computerized methods. In such studies, minimum and maximum amounts for these clearances are defined by key parameters such as the bearing operating temperature, minimum oil film thickness and maximum oil film pressure.
Where the bearing-to-shaft clearance is reduced, the oil film is subjected to a high shearing stress causing the sleeve bearings to overheat, besides facilitating a contact between the shaft and the bearing due to an excessively thin oil film. Under such conditions, the bearing material is subject to failure by overheating and excessive wear. On the other hand, large clearances cause an excessive hydrodynamic pressure in conjunction with a reduction in the oil film thickness, thereby exposing the bearing material to fatigue failure and excessive wear. The numerical techniques currently employed afford a prediction of acceptable limits for assembly clearances based on lubricating oil characteristics and the bearing geometry and materials.
The high-load four-stroke internal combustion engines, designed according to current trends, have subjected bearings, particularly the connecting rod bearings, to an excessive load and wear, which become prohibitive and eventually cause fatigue and overheating problems. These engines, in which high operating speeds are combined with a low mass interest of every component, subject the connecting rods to high inertia loads thereby causing distortions on their big end hole during operation. Due to the connecting rod configuration and the manner by which they transmit forces, the resulting distortions are such that during the exhaust stroke and, to a lesser degree, during the compression stroke, the rod larger end tends to have its diameter increased along the centerline of the big end and small end holes, and reduced in a direction orthogonal to the centerline. On the other hand, during the expansion stroke the big hole tends to have its diameter increased, or even unchanged, in a direction orthogonal to the connecting rod big end and small end centerline.
The amounts of these diametral variations are dependent on the inertia loads acting on the connecting rods as well as the structural resistance of the rods, which are related to both the quantity and distribution of mass in its design. The reduction in clearance in the direction of closing of the connecting rod hole causes disturbances in the creation of a stable lubricating oil film and a consequential increase of the operating temperature. Under such conditions, a bearing-to-journal contact may take place close to the point below the parting plane with a subsequent failure of the bearing material caused by excessive wear and overheating.
Because of the instability or turbulence developed in the oil film, this closing can further lead the bearing sliding surface to erosion damages caused by cavitation in the oil film. In the aforesaid circumstances of closing the hole under inertia load, one must increase the clearance in the direction of closing thereby enabling a stable lubricating oil film, thus preventing heat from being generated above levels determined beforehand. This measure, an increase of the diametral clearance by the use of a pair of conventional halfshells, although solving problems arising out of the closing of the rod big end hole subjected to inertia loads, can on the other hand give rise to problems on the bearing placed in the rod half during the expansion stroke. An excessive increase in the operating clearance may create a highly detrimental increase in the hydrodynamic pressure developed on the lubricating oil film, in conjunction with an equally detrimental decrease in the minimum oil film thickness. These two conditions may lead to an excessive wear or fatigue failure of the bearing material.